Temporarily allow failures on the docs shard (#34618)

https://github.com/flutter/flutter/issues/34435

.cirrus.yml

diff --git a/.cirrus.yml b/.cirrus.yml
index 6907b62..2458cf1 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -25,6 +25,8 @@
   setup_script: ./dev/bots/cirrus_setup.sh
   matrix:
     - name: docs
+      # TODO(flutter/flutter#34435): remove allow_failures
+      allow_failures: true
       env:
         SHARD: docs
         # For uploading master docs to Firebase master branch staging site